It would ideally open a browser page to the reference for whatever your mouse cursor is over in script, similar to how VisualStudio works.
I’ve seen mention in the knowledgebase of this functionality being used, but if I press F1 when mousing over reserved and defined words in script, it doesn’t seem to do anything for me.
UPDATE: keybinds for this action still fail most of the time and I don’t know why. At this point, I’m guessing it’s a bug in MonoDevelop. If I just use the menu Help->Unity API Reference, it launches the browser search 100%.
On a Mac, with the relevant method, property, or field highlighted (doubleclick whatever you want to search for), press command+‘. This is probably different on a PC, but it’s probably ctrl+’. You can likely reassign this hotkey in the monodevelop settings.
In the Mac is CMD ’ in more recent versions, or CMD + in older versions. Anyway, it wasn’t working on my computer (I think because of the language I use in my keyboard), so I changed it:
Go to Preferences > Environment > Key Bindings > Help (or just type “api” in the search box) > Unity API Reference, and change it to whatever you want.
In Unity 5.1 it is " ctrl + ’ " and can be changed from tools → options → Key bindings.